1. Technical Field
The present invention relates to a method of forming a conductive layer, a conductive layer formed using the method, an organic electroluminescent device including the conductive layer, and a method of manufacturing the organic electroluminescent device. More particularly, the invention relates to a method of forming a conductive layer that has high transmittance with respect to light having a wavelength of 300 nm to 700 nm and thermal-chemical stability, a conductive layer formed using the method, an organic electroluminescent device including the conductive layer, and a method of manufacturing the organic electroluminescent device.
2. Related Art
Organic light emitting devices (OLEDs) are self-emissive devices in which, when a current is provided to a fluorescent or phosphorescent organic layer, electrons and holes are combined in the organic layer, thereby emitting light. OLEDs are lightweight, and can be relatively easily manufactured using a few components. In addition, OLEDs can realize high-quality images, have wide viewing angles, and can perfectly realize moving pictures. Furthermore, OLEDs can realize high color purity and low power consumption with low operating voltage, and are therefore suitable for portable electronic devices.
OLEDs can be categorized into devices using a low molecular substance and devices using a polymer substance according to the characteristics of the material used, and the manufacturing process. In the process of manufacturing devices using a low molecular substance, vacuum deposition is used to form a layer, emission materials can be easily refined to a high degree of purity, and a color pixel can be easily obtained. However, there are several problems in the practical use of a low molecular substance in devices. For example, quantum efficiency should be improved, crystallization of a layer should be prevented, and color purity should be improved.
Meanwhile, studies on light emitting devices using polymer molecules are being carried out after a Cambridge group in 1990 reported that, when electricity was applied to poly (1,4-phenylenevinylene) (PPV), that is, a π-conjugated polymer, light was emitted. The π-conjugated polymers have a chemical structure in which single bonds (or s-bonds) and double bonds (or π-bonds) are alternated so that π-electrons are not polarized and move relatively easily along the bonding chain. Due to such a semi-conductive property of π-conjugated polymers, the entire range of visible light corresponding to a HOMO-LUMO band-gap can be easily obtained through appropriate molecular design when the π-conjugated polymers are used in an emission layer of an organic electroluminescent device. In addition, a layer can be easily formed by spin coating or printing, and thus, the manufacturing process is simple and inexpensive. Furthermore, the layer formed has excellent mechanical properties because the polymer molecules have a high glass transition temperature. Accordingly, in the long term, it is expected that polymer light emitting devices will have a commercial competitive edge over low molecular light emitting devices.
Generally, these polymer light emitting devices include, as an organic layer, a multi-layer which includes a hole injection layer formed using a conductive polymer, an emission layer, and an electron injection layer, instead of a single emission layer, to improve efficiency and decrease operating voltage.
Such a multi-organic layer can be basically divided into a hole related layer, an electron related layer, and an emission layer.
A hole injection layer (HIL) is generally formed by spin coating a PEDOT[poly(3,4-ethylene dioxythiophene)]-PSS[poly(4-styrenesulfonate)] aqueous solution (Model: Baytron-P, commercially available from Bayer AG Inc) on an indium tin oxide (ITO) electrode in the process of manufacturing an organic light emitting device.
However, a conductive layer which is formed through conventional wet polymerization of monomers that make up a conductive polymer has low transmittance and low conductivity due to the presence of impurities generated during a process of preparing a polymer solution.
Accordingly, there is a need to develop a uniformly conductive, transparent layer in order to obtain an OLED having high efficiency and a long lifetime.